1. Field
Example embodiments relate to a Vertical External Cavity Surface Emitting Laser (VECSEL), and more particularly, to a simple, compact VECSEL having low manufacturing costs.
2. Description of the Related Art
Generally, vertical cavity surface emitting lasers (VCSELs) emit a laser beam in a direction perpendicular to a substrate. A VCSEL has a high coupling efficiency because it can oscillate in a single longitudinal mode with a narrow spectrum bandwidth and emit a beam having a small angle of divergence. Furthermore, since a conventional VCSEL can be easily integrated into other devices due to its structure, the conventional VCSEL may be used as a pumping light source. However, it is more difficult for the VCSEL to oscillate at a single transverse mode than edge emitting lasers because the laser beam from the VCSEL has a multimode state due to thermal lens effect induced by increased output power. Furthermore, the conventional VCSEL generally requires a narrow oscillation region for single traverse mode operation, thereby resulting in a low output power.
A conventional vertical external cavity surface emitting laser (VECSEL) provides the above-described advantages of a VCSEL while also realizing high power output. A conventional VECSEL has a gain region that is increased by replacing an upper mirror of the structure formed on a corresponding substrate with an external mirror. The external mirror is a mirror which is formed to be separate from the structure, and thus can obtain a high output power.
FIG. 1 schematically illustrates the configuration of a conventional VECSEL 10 using front optical pumping. Referring to FIG. 1, the VECSEL 10 includes a semiconductor chip 13 having a Distributed Bragg Reflector (DBR) layer 11 and an active layer 12 sequentially stacked on a heat sink 14, a pump laser 15 disposed in front of the semiconductor chip 13 and providing a pump beam, an external mirror 20 separated by a distance from the laser chip 13 and facing the semiconductor chip 13, and a lens 16 disposed between the pump laser 15 and the semiconductor chip 13 and focusing the pump beam emitted by the pump laser 15.
The VECSEL 10 may further include a Second Harmonic Generation (SHG) crystal 18 and a birefringent filter 17 disposed between the active layer 12 and the external mirror 20. The SHG crystal 18 may induce forming of light having double the frequency of a primary light exited in the active layer 12 and the birefringent filter 17 may increase SHG efficiency. Linear polarization and narrow longitudinal mode line-width are generally required to increase light conversion efficiency. The birefringent filter 17 may induce a narrow line-width and aid wavelength selection to increase light conversion efficiency.
The active layer 12 may have a multi-quantum well structure with Resonant Periodic Gain (RPG) and may be excited by a pump beam to emit a laser beam having a wavelength λ2. The pump laser 15 may irradiate a pump beam having a shorter wavelength λ1 than the primary light emitted from the active layer 12 onto the active layer 12 in order to excite the active layer 12.
In the above-mentioned conventional structure, if a pump beam emitted from the pump laser 15 at the wavelength λ1 is incident on the active layer 12, the active layer 12 may be excited to generate light having a specific wavelength λ2. The beam generated by the active layer 12 may be repeatedly reflected between the DBR layer 11 and the external mirror 20 to repeatedly pass through the active layer 12. A portion of the beam amplified in the active layer 12 may be emitted out of the VECSEL 10 through the external mirror 20. The beam exiting the active layer 12 may be filtered by the birefringent filter 17 to realize a narrow line-width, single longitudinal mode beam. The filtered beam may be converted into a beam having a different wavelength by the SHG crystal 18. For example, the SHG crystal 18 may convert a laser beam having an infrared wavelength range into a laser beam having a visible wavelength range.
The birefringent filter 17 must be oriented at a desired angle, i.e., Brewster's angle to a main path of a beam in order to select the polarization state and wavelength of a resonating beam. Furthermore, the birefringent filter 17 may require a jig for alignment according to the polarization direction, and thus the VECSEL 10 may be bulky. Thus, the conventional VECSEL 10 may not be suitable for a compact laser source in blue-green light source applications, for example. Furthermore, the birefringent filter 17 requires a complicated manufacturing process, thus resulting in high manufacturing costs.